Cardiac sympathetic nerves, in contrast, facilitate AV conduction. They decrease the AV conduction time and enhance the rhythmicity of latent pacemakers in the AV junction. The norepinephrine released at the postganglionic sympathetic nerve terminals increases the amplitude and slope of the upstroke of the AV nodal action potentials, principally in the AN and N regions of the node.
The bundle of His passes subendocardially down the right side of the interventricular septum for approximately 1 cm •Fig. 16.22 Effects of a Brief Vagal Stimulus (St) on the Transmembrane Potential. Thisresponsewasrecordedfromanatrioventricular(AV)nodalfiber(red trace). Compoundactionpotentialswererecordedfromtheatrium(A1,A2,A3;upper blue trace)andfromtheHis-Purkinjesystem(H;lower blue trace).Notethatshortlyaftervagalstimulation,themembraneofthefiberwashyperpolarized.Theatrialexcitation(A2)thatarrivedattheAVnodewhenthecellwashyperpolarizedfailedtobeconducted,asdenotedbytheabsenceofadepolarizationintheHiselectrogram(H).Theatrialexcitationsthatpreceded(A1)andfollowed(A3)excitationA2wereconductedtotheHisbundleregion.(RedrawnfromMazgalevT.etal.Am J Physiol. 1986;251:H631.) and then divides into the right and left bundle branches (see
Fig. 16.17 ). The right bundle branch, a direct continuation of the bundle of His, proceeds down the right side of the interventricular septum. The left bundle branch, which is considerably thicker than the right, arises almost perpendicular from the bundle of His and perforates the interventricular septum. On the subendocardial surface of the left side of the interventricular septum, the left bundle branch splits into a thin anterior division and a thick posterior division.
Conductionofimpulsesintherightorleftbundlebranchorineitherdivisionoftheleftbundlebranchmaybeimpaired.Conductionblocksmaydevelopinoneormoreoftheseconductionpathwaysasaconsequenceofcoronaryarterydiseaseordegenerativeprocessesassociatedwithaging,andtheygiverisetocharacteristicECGpatterns.Blockofeitherofthemainbundlebranchesisknownasright orleft bundle branch block. Blockofeitherdivisionoftheleftbundlebranchiscalledleft anterior orleft posterior hemiblock.
The right bundle branch and the two divisions of the left bundle branch ultimately subdivide into a complex network of conducting fibers, called Purkinje fibers, that spread out over the subendocardial surfaces of both ventricles. Purkinje fibers have abundant, linearly arranged sarcomeres, as do myocytes. However, the transverse (T) tubular system, which is well developed in myocytes, is absent in the Purkinje fibers of many species. Purkinje fibers are the broadest cells in the heart: 70 to 80 µm in diameter, in comparison with diameters of 10 to 15 µm for ventricular myocytes. Partly because of the large diameter of the Purkinje fibers, conduction velocity (1 to 4 m/second) in these fibers exceeds that in any other fiber type within the heart. The increased conduction velocity enables rapid activation of the entire endocardial surface of the ventricles.
The action potentials recorded from Purkinje fibers resemble those of ordinary ventricular myocardial fibers. However, because of the long refractory period of Purkinje fiber action potentials, many premature excitations of the atria are conducted through the AV junction but are then blocked by the Purkinje fibers. Blockade of these atrial excitations prevents premature contraction of the ventricles. This function of protecting the ventricles against the effects of premature atrial depolarization is especially pronounced at slow heart rates because the action potential duration, and hence the effective refractory period, of the Purkinje fibers vary inversely with the heart rate (see